It's a problem to rent a flat because there are many hotels in the town.

Nice idea. Your sentence is understandable but a bit awkward. Here are some smoother alternatives you can use, depending on the tone you want:

• It is difficult to rent a flat because there are many hotels in town.
• Finding a flat to rent is difficult, given the number of hotels in town.
• The abundance of hotels in town makes renting a flat challenging.
• There are too many hotels in town, which makes it hard to rent a flat.

Notes:

• Use “in town” rather than “in the town” for a more natural sound in most contexts.
• “It’s a problem to rent a flat” sounds odd; “It’s difficult to rent a flat” or “Finding a flat to rent is difficult” flows better.

If you want a formal option for an essay: “The presence of numerous hotels in town complicates the process of renting a flat.”

Would you like me to adapt this to a specific exam style or expand it into a short paragraph?